In general, there are four main methods of measuring alternating voltages in high and medium voltage transmission and distribution systems, namely: magnetic methods in which potential transformers, for example, are used; methods utilising a potential divider or tap using impedances; optical methods utilising Pockels cells and liquid crystals etc; and mechanical methods utilising field mills, vibrating reeds etc. In addition, such methods can be combined to provide further methods of measuring high and medium alternating voltages.
Voltage measurements in high-voltage systems are traditionally made using potential transformers or capacitively-coupled potential transformers. These devices are both large and expensive, precluding their wide-spread use in many applications. In addition, as both these types of transformers require a direct connection to the high-voltage conductor, extensive safety precautions need to be put into place to ensure the necessary isolation requirements are provided.
Voltage dividers, either resistive or capacitive, are a second class of devices used when less measurement precision is acceptable. Resistive voltage dividers have the disadvantage that a galvanic connection to the high-voltage conductor is required. Capacitive voltage dividers can, however, be implemented so that the required isolation to the high-voltage is already provided by the existing installation.
Capacitive methods for measuring the potential on high-voltage cables are known.
In DE-A-3702735, a method is described for constructing a capacitive element and a capacitive voltage divider on a cable so that the line voltage at any location along a cable system can be measured. In this method, the dielectric strength of a high-voltage capacitor is determined by the conductor insulation of the cable itself, and, the voltage divider can subsequently be installed by applying specially shaped connecting sleeves or cable terminations at any point in a cable network to allow continuous voltage measurement at any number of test points in a cable network.
This method, however, has the disadvantage that the electric field within the cable may be perturbed by the added structures and its service life compromised due to the absence of semiconducting layers present in modern state of the art medium voltage cables. [By the term “semiconducting layer” is meant, as is well known in the art, a resistive material, typically, a carbon-laden polymer. In some cases, it may be a non-linear material whose resistivity decreases with increasing voltage.] Modern high-voltage cables use such layers to prevent electric field discontinuities in the cable and thus improve reliability and enable a thinner dielectric to be used. Furthermore, the method is based on a full capacitive divider, and, therefore provides a voltage based output. Moreover, such methods are extremely sensitive to moisture contamination, or any other effect that adds, even very small amounts, of conductivity to ground. This means that, even slight contamination, results in significant errors in any measurements taken. This makes the integrity of mechanical construction critical to maintain stability over the life of the device.
In U.S. Pat. No. 5,051,733, a method is described for constructing a capacitive element on an existing cable for sensing stray electric fields within the vicinity of a high voltage circuit. The capacitive element comprises the addition of a semiconducting layer and a contact layer to the semiconducting layer over an existing cable. The arrangement is used for high voltage circuits inside a mine power centre or switch house that are connected by means of individual insulated conductors. These insulated conductors are typically unshielded by any surrounding conductive layer, and, as a result, electric fields associated with high voltage energisation of the conductors extends beyond the cables themselves to other phase conductors and surrounding grounded surfaces within the power centre enclosure. Here, these stray electric fields are used to provide a visual indication that the high voltage conductors are energised. In effect, a capacitor is constructed around the insulated conductor and this provides a high impedance circuit through a gas discharge lamp to ground. The current through the capacitor is sufficient to cause the discharge tube to glow when the high voltage circuit is energised, thus providing a visible warning to maintenance personnel.
This method, however, has the disadvantage that the layers of semiconducting and contact materials need to be added to pre-existing cables. In addition, no grounded electrostatic screen is incorporated into the system. Due to this, interference will be present due to neighbouring equipment and/or other conductors. Moreover, the addition of layers can create issues with the accuracy and stability of the overall construction, making the long-term stability of the arrangement unpredictable. Furthermore, the arrangement described is intended for providing an indicating device not for voltage measurements.
U.S. Pat. No. 5,065,142 describes a method of constructing a capacitive element over an existing cable that is similar to that of U.S. Pat. No. 5,051,733. Flared sections of the conductor are provided to produce a gentle electric field distribution at the edges. A capacitor and a neon bulb or other gas discharge bulb characterised by avalanche breakdown at a predetermined voltage are connected in parallel with a rectifier. Optionally, a piezoelectric or other sound-generating device may be in series with the neon bulb. When the central conductor wire is energised at 1000 volts AC or above, the bulb flashes, and, the optional sound generator activates, both at a frequency dependent on the length of the external sheath along the insulation.
U.S. Pat. No. 4,241,373 describes a switchgear assembly which includes a vacuum interrupter, a current transformer, and a capacitive voltage sensor, all embedded within a cast epoxy housing mounted to a solidly grounded support structure. This has the disadvantage that it requires the unit to be manufactured as part of the cast body of the device, including a high-voltage conductor with its associated isolation, and a separate capacitive element cannot be implemented at a later date.
A circuit condition monitoring system for an electrical power distribution system is described in U.S. Pat. No. 4,794,331. The distribution system includes a connector component having an integrally formed test point which provides fault current or voltage loss monitoring of a conductor within the connector component. The connector component includes a circuit monitoring module that can be capacitively coupled to the system conductor and to the module to function as a test point for providing operating power to the module. A sensing plate is embedded in the connector body to provide the capacitive coupling. This arrangement, however, has the disadvantage that its implementation in an existing installation requires the replacement or addition of cable connectors and sockets.
An indicator device that indicates a circuit condition is described in U.S. Pat. No. 4,794,329. The indicator device detects the occurrence of a fault current in a high voltage conductor from which the indicator device is suspended. The device includes a housing having at the upper end thereof a pair of outwardly projecting cable engagement members. The engagement members are formed of a resilient insulating material and each includes an outwardly projecting base portion and an inwardly projecting end portion which engages and holds the cable against the rear wall of the housing. Operating power for the indicator device is derived from the potential gradient of the electric field surrounding the conductor by means of a metallic plate positioned within the housing adjacent the conductor, and a metallic ring and electrically conductive coating within the lower end of the housing.
In this arrangement, the mechanical stability is limited and no electrostatic screening is provided, thus interference and proximity effects with neighbouring structures will be significant. The device is not used for determining measurements but purely for providing indication of the presence of a fault current in the high voltage conductor on which it is mounted.
In U.S. Pat. No. 3,538,440, a method is described for constructing a capacitive element inside an existing cable which comprises an electrode placed below the surface level of the cable dielectric. This has the disadvantage that the cable dielectric must be compromised during the addition of the capacitive element. In many cases, the cable dielectric is already thin, limiting the applicability to certain cases where such a disturbance in the dielectric can safely be made. Essentially, the method relates to a means of keeping the output voltage constant, which is undesirable for a measuring device.
A high voltage measuring device is described in U.S. Pat. No. 4,121,154 that is used to measure the amount of voltage in an alternating current carrying line. The measuring device uses a capacitive element with an associated amplifier which, in use, is brought near to the high voltage line. A voltage output is provided which is derived from a dc signal that has been converted from an ac signal. However, this device is not suitable for use with many cable installations and its accuracy depends strongly on environmental factors disturbing the electrical field that is sensed by the device.
In U.S. Pat. No. 4,052,665, a method is described for constructing a capacitive element on an existing cable which comprises of concentric electrodes clamped around the cable. A capacitive pickup device is clamped to an insulated conductor and a measurable voltage is derived which is a linear function of the magnitude of pulsating high voltage in the conductor, the high voltage being of the order of 15 to 40 kilovolts such as encountered in ignition systems of internal combustion engines.
This method, however, has the disadvantage that the clamp does not manage the electric field at its edges and may cause dielectric integrity problems over time when the device is permanently connected to the insulated conductor.
Known capacitive voltage measurement methods fail in practice due to two factors. On one hand, voltage divider-based capacitive methods are highly susceptible to contamination, and on the other hand, modern cables use thin dielectrics and rely on field gradient control to keep the internal electric fields free from discontinuities and prevent breakdown of the dielectric.